Not gay or bisexual, but in between bi and straight. Like, being straight, but you would consider doing something with a member of the same gender should the situation arise. Mainly for pleasure and adventurous reasons, you probably wouldn't date them. Also can be called bent.
It DOESNT mean gay as some people would say it does.

I didn't think sarah was hetero-flexible until she gave me a lap dance and then went off to make out with her boyfriend.

A person who enjoys sex with both male & females but considers themself heterosexual in the sense that they do not relate with the same sex for an emotional relationship like a bi-sexual could. Heteroflexible is a heterosexual person that is beyond the bi-curious stage but would never engage in an emotional relationship with the same sex.

I like the fact that my wife is heteroflexible, she really likes girls as an appeitizer before she's ready for the main course!